Jeskai Baller’s first ability will resolve before Jeskai Baller does. If Jeskai Baller is countered or otherwise leaves the stack in response to that triggered ability, the triggered ability will still resolve as normal.

If you cast Jeskai Baller from your hand and the spell resolves, you must exile the spell due to its rebound ability. You cannot choose to have it enter the battlefield as a creature.

If you cast Jeskai Baller from any zone other than your hand (including from exile using its rebound ability) and the spell resolves, Jeskai Baller enters the battlefield as a creature.

Casting the card again due to rebound’s delayed triggered ability is optional. If you choose not to cast the card, or if you can’t because an effect prohibits it, the card will stay exiled. You won’t get another chance to cast it on a future turn.

If a spell with rebound that you cast from your hand doesn’t resolve for any reason, including being countered, the spell will be put into its owner’s graveyard and you won’t get to cast it again on your next turn.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.